The invention concerns a continuously variable transmission for a vehicle comprising one variator with a non-positive, special friction-type power transmission such as a friction gear with interacting toroidal treads disposed in pairs between which roller bodies rotate, or a conical disk continuously variable transmission and a first planetary gear. The transmission input power can be divided in at least two performance ranges wherein, in one performance range, the power can be transmitted either by a first power split from a transmission input via the variator, the same as the first planetary gear, to a transmission output or by a second power split from the transmission input via a mechanical split by-passing the variator to the first planetary gear and the transmission output, or power split by means of both power splits.
Such a friction gear is already known from the Applicant""s WO 98/34051. The problem to be solved by the invention was to improve, with regard to total efficiency achievable, a transmission operated in two performance ranges and, furthermore, to be able to adapt the transmission with basically the same design to different installation conditions. In the second performance range, the transmission input power is transmitted to the output shaft with a power split in the transmission.
In such a continuously variable vehicle transmission, a reduction of the torque in the variator is proposed only in one performance range as it is known from the prior art. From the field of hydrostatic transmissions, power split transmissions, which disadvantageously have a strong noise development with poor efficiency and also high costs of production, are further known.
The problem to be solved by this invention is to provide a continuously variable vehicle transmission which overcomes the above mentioned disadvantages and especially makes a reduction of the variator load over the whole performance range and an improved efficiency possible.
The problem is solved, according to the invention, in a general vehicle transmission by the fact that the transmission power is split both in the first and in the second performance ranges and can be transmitted from the transmission input to the transmission output, via the first and the second power splits. By virtue of such a split power transmission in the first and in the second performance ranges, a continuously variable vehicle transmission results which advantageously makes a small variator load in both performance ranges possible in comparison to a transmission without a power split or to a transmission which has power split in only one performance range. A better efficiency of the continuously variable vehicle transmission is further obtained by the power split. Compared to the transmission without a power split, a greater transmission spreading is advantageously made possible.
In one special development of the invention, it is proposed to provide a second planetary gear, for power split in both performance ranges, and then to add up again in the first or the second planetary gear the two power portion which have been passed via the first and second power splits. It is thus advantageously obtained, a simple and economical means of implementing the power split in both performance ranges of the above mentioned vehicle transmission.
At least one of the two planetary gears is advantageously situated in the power flow between the transmission input and the transmission output parallel to the variator or also located downstream of the variator.
In one development, both planetary gears are disposed in the power flow parallel to each other between the transmission input and the transmission output.
The first and the second planetary gears are advantageously disposed so that in the first performance range the variator is connected with the sun gear of the first planetary gear, the transmission input with the planet carrier and the transmission output with the ring gear of the first planetary gear.
In addition, the first and the second planetary gears are disposed so that in the second performance range, the variator is connected with the ring gear of the second planetary gear, the transmission input with the sun and the transmission output with the stem of the second planetary gear.
In a preferred design of the invention, at least two clutches are provided for connecting the two planetary gears which are situated in different places in the transmission according to the utilization of the transmission or to possibilities of installation.
Both clutches can be shifted so that one planetary gear can be loaded with power and the other planetary gear rotate along idlely without power transmission.
In one development, both clutches are situated in the mechanical power split parallel to the variator.
To shift a reverse gear, one brake parallel to both clutches is advantageously provided according to the preceding design.
In one other development, it is proposed that both clutches be provided in the power split of the variator and be located downstream thereof. Thereby both clutches are advantageously loaded with low torque.
In another development, both clutches are situated between the two planetary gears and the transmission output.
In a special development of the invention, in the first performance range, the vehicle is driven out of a neutral geared position of the transmission both in forward and in reverse. For driving in geared neutral, no other driving element with a slip regulation, such as a converter or a friction clutch, is basically required. Besides, an additional reverse gear is not necessary, since it results from the ratio of the continuously variable transmission.